External IMAP account gives strange recursive reading of Trash

Could be that this is the wrong forum. But maybe not: while migrating from other mail servers many people end up fighting IMAP problems.
Here's a new one for you.

ZCS up and running nicely, so I tried to sniff on the excellent feature to add an external IMAP account in the web interface. Adding one IMAP account from a quite well known provider went well so I tried to add another less known provider (IP phone company). Test of the account went well but when I clicked "Save" this is what hit me: